This application claims priority under 35 U.S.C. §§ 119 and/or 365 to Patent Application Ser. No. 102 18 630.8 filed in Germany on Apr. 25, 2002 the entire content of which is hereby incorporated by reference.
The present invention relates to a disc-milling cutter, especially for milling crankshafts, and a cutter insert for use therein. A disc-milling cutter comprises a milling disc on which cutter inserts are mounted.
Milling is a material-removing process wherein the tool rotates, and relative movements between the workpiece and the tool take place perpendicular to the rotational axis of the tool. The tool is fitted with cutters, which produce the cutting motion during the rotation of the tool about the central tool axis. Feeding movements can be made in various directions. Such feeding movements may be performed either by the tool or by the workpiece or even by both. By way of distinction from turning or drilling, the cutters are not constantly engaged with the workpiece. On the contrary, after one cut on the workpiece, the cutters are returned to the starting position. This provides the advantage that the cutters can cool down and material chips can be removed from the chip spaces.
In the context of milling, cutter inserts, which provide the appropriate cutting edges, are generally attached to the tools used. Cutter inserts of this kind, which are also referred to as tool tips, have been known in principle for a considerable time.
By “crankshafts” is generally meant machine elements which convert linear movements into rotating movements or vice versa. Forging or casting methods are generally used in the mass production of crankshafts. Modern engine design places increasing demands on the performance and smooth running of crankshafts. Costs must also be kept to a minimum. Essential requirements include a higher engine performance for a given piston stroke, an improved level of efficiency, that is, reduced loss due to friction, and a reduction in weight. This means that all design parameters, such as dimensions, material properties and surface treatment, in particular low surface tolerances, must be optimized. At the same time, in order to reduce production costs during the manufacturing of crankshafts, grinding has frequently been replaced by material removal with a defined cutting edge, for example, turning, turn broaching or milling, wherever possible.
In particular for the machining of crank bearings on crankshafts, external milling using the pendulum process has recently proved successful. In this context, a distinction is made between rough-milling and finishing milling; milling cutters with a diameter of 700 mm and up to 270 indexable inserts have been widely used.
In the context of finishing milling, the milling cutters are generally fitted with diameter cutters and recess cutters. The diameter cutters produce the diameter of the bearing, which will be ground to a finish in a subsequent operation, and the recess cutters produce the finished bearing width, that is, the opposing surfaces of the stop collar (also called the oil collar) for the connecting rod, and the recess which is used as a grinding outlet in the transitional region between the bearing neck and the stop collars mentioned above on the radially internal portion of the crank webs.
By way of example, FIG. 1 shows a crankshaft 5 in the mounted condition. The crankshaft 5 has main bearings A, B, C, B and E and crank bearings 1, 2, 3, 4. The individual crank bearings are processed by means of the disc-milling cutter 6 using the pendulum process. In the context of the pendulum process, the crankshaft rotates around its axis of rotation during machining. The disc-milling cutters 6 rotating at the desired cutting speed follow the corresponding movement of the crank bearings 1, 2, 3,4. The processing of the crank bearings 1,2, 3,4 of the crankshaft 5 is essentially completed after each complete rotation of the crankshaft 5. The circumferential surfaces of the crank webs can also be processed in the same operation.
FIG. 2 shows an enlargement of the machining of the crank bearing 3 from FIG. 1. In this case, the disc-milling cutter 6 is fitted with diameter cutters 7, arranged in a cross-toothed manner, and recess cutters 8. The diameter cutters 7 are arranged tangentially, while the recess cutters 8 are arranged laterally. The known laterally-arranged recess cutters are designed as an indexable insert with four cutting edges, which engage with the workpiece in dependence upon the direction of installation.
In particular when connecting rods are guided axially through a crankshaft, considerable demands are also placed on the surface of the stop collar and/or oil collar 27 of the crankshaft 5 in the meantime, so that these must be manufactured with particular care. Current disc-milling cutters have an optimum run-out tolerance of approximately 20 μm, so that the stop collars 27 can generally not be manufactured using milling processes alone, but also require subsequent grinding or precision turning.
WO 01/76796 proposes a cutter insert for camshaft milling cutters. In case of a camshaft milling cutter, the cutting edge of the cutter insert provides several cutting edge portions with an initial, straight portion, a second, straight portion adjoining the first portion and enclosing with it an angle between 90° and less than 180°, and a third cutting edge portion providing a convex curved course. The convex portion in this context forms the recess cutter. However, this camshaft milling cutter also has run-out tolerances in the order of magnitude of 20 μm.
The object of the present invention is therefore to provide a cutter insert and a milling cutter, which is suitable for the increasing demands of engine design and, in particular, which allows a significantly reduced run-out tolerance, so that the surfaces of the stop collars on crankshafts can also be processed in one milling operation without follow-on grinding. Moreover, the object of the present invention is to provide a cutter insert, which can be used in a cost-favorable manner for this purpose.